A radio transmitter 100 known in the art is illustrated in FIG. 1. It comprises a digital signal source 102, which typically generates the information bearing signals in the baseband domain, (i.e. the in-phase-signal and the quadrature-signal), a compensation unit 104 for compensating the non-linear distortions, two digital-to-analogue converters (DACs) 106, 108, one for the in-phase-signal and one for the quadrature-signal, an analogue modulator 110, to modulate the baseband signal onto a radio frequency (RF) carrier, an amplifier 112 and an antenna 114.
The modulation is often performed in two steps. In the first step the information bearing signal is modulated onto an intermediate-frequency (IF) carrier. The frequency of such a carrier is much below the radio frequency. In the second step the IF signal is converted to the radio frequency. Typically, such a configuration is used, when the bandwidth of the information bearing signal is quite large.
In other solutions known in the art, the modulation could be performed digitally, using a low IF. In these types of solutions only one DAC is required. However, the sampling frequency must be at least twice as large compared to analogue modulation. This is why digital modulation is only applicable if the bandwidth of the information bearing signals is relatively small.
Although analogue modulators can be used for wide bandwidth signals, they always suffer from imbalances. The problem with compensation of the imbalances of the modulator is that when the feedback-signal is down-converted into baseband it is impossible to distinguish between the imbalances of the transmit site modulator and the feedback part. A digital modulator, on the contrary, can be build as precise as desired. This leads to a situation that for radio signal transmitters operating on a wide bandwidth it is necessary to use analogue and imbalanced modulators and digital modulators (balanced modulation) can be used only in transmitters operating on a small bandwidth.
In order to compensate distortions introduced by the modulator and/or amplifier it is necessary to monitor the radio signal affected by these distortions.
Hence, an improved apparatus and method for monitoring non-linear distortions of radio signals would be advantageous and in particular one, which allows for monitoring that uses low rate sampling of the signal to be monitored.